Keep in mind that nearly all of the materials, construction methods and pipe sizes can be substituted for something else. It's up to you to decide what kind of materials you'd like to work with and make individual adjustments to the design accordingly.
Also, you'll need to make your own measurements so don't expect them in this writeup.
One of the biggest problems with doing a pump action setup in a SNAP is that the available distance you have in the front of the gun is much smaller than the stroke of the priming handle. By using part of the barrel and hopper assembly to form the base of your priming rail, you are given a much greater distance to work with without actually altering the basic design of the SNAP.
Here is the slide rail, which consists merely of a 1/2" T, an elbow (I used a 1/2" CPVC street elbow) and a length of pipe which will slide smoothly inside of your foregrip. The priming rail should also be elevated in such a way that the slide fits comfortably underneath the plunger tube. You will also need an attachment site near the other end of the blaster.
And here is the foregrip:
Again, it can be constructed in a variety of methods and more durable materials but the concept is the same. I used 3/4" CPVC built to 1" PVC, wooden dowels, a wooden board, epoxy putty for structural support and 1" screws. The idea was to make a simple slide just to see if it would work without the spring, but despite the shoddy construction can fully compress a [k26] without visible stress marks. Even so, I will probably make another one with different stuff.
And just to illustrate how simple this modification actually is, by popping off the front 1/2" T and unscrewing the endcap at the end of the plunger, you can easily remove the entire assembly, returning to a pull-back setup.
Edited by Fome, 30 September 2010 - 02:25 PM.